Zeolites are crystalline aluminosilicate compositions which are microporous and which are formed from corner sharing [AlO4/2]− and SiO4/2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents (SDAs) such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces of the zeolite as well as on internal surfaces within the pores of the zeolite.
In 1982, Wilson et al. developed aluminophosphate molecular sieves, the so-called AlPOs, which are microporous materials that have many of the same properties of zeolites, but are silica free, composed of [AlO4/2]− and [PO4/2]+ tetrahedra (see U.S. Pat. No. 4,319,440). Subsequently, charge was introduced to the neutral aluminophosphate frameworks via the substitution of SiO4/2 tetrahedra for [PO4/2]+ tetrahedra to produce the SAPO molecular sieves (see U.S. Pat. No. 4,440,871). Another way to introduce framework charge to neutral aluminophosphates is to substitute [M2+O4/2]2− tetrahedra for [AlO4/2]− tetrahedra, which yield the MeAPO molecular sieves (see U.S. Pat. No. 4,567,039). It is furthermore possible to introduce framework charge on AlPO-based molecular sieves via the introduction both of SiO4/2 and [M2+O4/2]2− tetrahedra to the framework, giving MeAPSO molecular sieves (see U.S. Pat. No. 4,973,785).
Among the SAPO molecular sieves disclosed in U.S. Pat. No. 4,440,871 is a species designated SAPO-17, which has the ERI topology (see DATABASE OF ZEOLITE STRUCTURES, www.iza-structure.org/databases). Several different routes to SAPO-17 were disclosed using the amine SDAs quinuclidine (Ex. 25, U.S. Pat. No. 4,440,871) and cyclohexylamine (Ex. 26, U.S. Pat. No. 4,440,871), which gave a small amount of Si substitution, the cyclohexylamine SDA yielding the composition (cyclohexylamine)0.245Al1.33Si0.048P. Similarly, SAPO-17 was also prepared using exclusively the hexamethonium SDA by Valyocsik and von Ballmoos (see U.S. Pat. No. 4,778,780).
More recently, Lewis et al. developed high phosphate solution chemistry leading to higher charge density SAPO, MeAPO, and MeAPSO materials, enabling greater substitution of SiO4/2 and [M2+O4/2]2− into the framework for [PO4/2]+ and [AlO4/2]−, respectively, using the ethyltrimethylammonium (ETMA+) and diethyldimethylammonium (DEDMA+) SDAs. These materials include ZnAPO-57 (U.S. Pat. No. 8,871,178), ZnAPO-59 (U.S. Pat. No. 8,871,177), ZnAPO-67 (U.S. Pat. No. 8,697,927, and MeAPSO-64 (U.S. Pat. No. 8,696,886). The relationship between the increasing product charge densities and reaction parameters, namely the ETMAOH(DEDMAOH)/H3PO4 ratios, were outlined in the literature (see MICROPOROUS AND MESOPOROUS MATERIALS, 189, 2014, 49-63).
Applicants have now synthesized a new family of charged silicometallophosphate framework materials with even higher charge densities than those mentioned above, designated SAPO-79. The SAPO-79 materials are the first alkali-stabilized microporous phosphates to have the ERI topology (see DATABASE OF ZEOLITE STRUCTURES, www.iza-structure.org/databases) and are synthesized in a mixed organoammonium/alkali SDA system, for example DEDMA+/K+. The utility of alkali in SAPO-based systems is uncommon and is required here to achieve the higher charge density. The SAPO-79 materials contain “Si islands” and are thermally stable to at least 500° C.